You are a security guard who accompanied a group of space colonists on their trip on the huge space ship, the Marathon. Suddenly, aliens who call themselves Pfhor attack the ship and massacre the colonists. What's worse, the aliens damage the omniscient super-computers on Marathon. One of them is heavily damaged, and the other becomes insane! Caught in the midst of the battle between the two AIs, trying to stay alive in the onslaught of the Pfhor, you begin the desperate fight for survival... In this first-person shooter, you have a variety of weapons that serve you in your battle against your enemies. Different types of enemies (organic or mechanical) take different amounts of damage depending on the weapon. You can also fire from two weapons (such as pistols) simultaneously. Shooting enemies is not the only thing you will do in "Marathon". It is possible to communicate with the damaged AIs of the ship by looking at messages they send you via computers. The story is unraveled...